BAC of .49.

[arnisador]

R.I. police say man had 0.491 blood alcohol level

State police say they arrested a man early Tuesday whose blood alcohol level was 0.491 percent — the highest ever recorded in Rhode Island for someone who wasn't dead.
[...]
The legal limit in Rhode Island is 0.08. A level of 0.30 is classified as stupor, 0.4 is comatose and 0.5 is considered fatal, according to the health department.

I'm pretty sure I went to college with that guy (and his frat brothers).

 

[Archangel M]

Personally seen a "career drinker" at .5. He was still talking and able to walk. He died a few years back. Body can only take so much of that sort of treatment.

 

[jks9199]

I've seen a .43... but the medics took him away before I could arrest him. And I've had a couple in the .3+ range who were amazingly lucid and functional. Career drinkers there...

There's a lot of individual variation in tolerance and visible or obvious impairment... In fact, sometimes it's situational. But the functional impairment is consistent, even in someone with a high apparent tolerance for alcohol.